As world leaders searched for a way out of Europe's mounting debt crisis, U.S. investors moved to the sidelines.The major market indexes closed modestly higher, after wavering between slight gains and losses throughout the morning. Trading volume was light and the stock moves were small. In Europe, markets were mixed.The Dow Jones industrial average rose 26.49 points, or 0.2 percent, to 12,127.95. It traded within a range of 75 point...